JSFiddle - React, Tailwind, and code Playground

HTML

<h1>Sub-total Calculation</h1>

	<form id='calcForm'>
		<input type="text" id="fld" placeholder="Enter in subtotal"/>
		<button type="submit" >Calculate</button><br> 
		<span id="sub-total"></span><br> 
		<span id="tax"></span><br> 
		<span id="total"></span><br> 
	</form>

JavaScript

window.addEventListener('load',function()
{
    var input = document.getElementById('fld'),
        subTotal = document.getElementById('sub-total'),
        tax = document.getElementById('tax'),
        total = document.getElementById('total'),
        subTotalCalc,
        toDecimal = function(num, precision)
        {
            precision = precision || 2;
            num = +(num || 0);
            return (Math.round(num*Math.pow(10,precision))/Math.pow(10,precision)).toFixed(precision);
        };
    document.getElementById('calcForm').addEventListener('submit',function(e)
    {
        e = e || window.event;
        e.preventDefault();//prevent form's default behaviour
        e.stopPropagation();//if submit was sent by clicking submit button, stop the event here
        if (input.value === '' || input.value != +(input.value))
        {
            alert('Please enter valid subtotal (numeric)');
            return;
        }
        subTotalCalc = toDecimal(input.value / 1.06);
	    subTotal.innerHTML = "Subtotal:" + " " + "$" + subTotalCalc;
		tax.innerHTML = "Tax:" + " " + "$" + toDecimal(input.value - subTotalCalc);
		total.innerHTML = input.value;
    },false)
},false);